chore: prepare v0.8.1 runbook release

[Maples7]

Summary

• bump VibeChard.version to 0.8.1
• add the 0.8.1 changelog entry for vch runbook reports missing Homebrew doc path #108
• change release automation to sync the full repository formula into maples7/homebrew-tap before writing the release URL/version/SHA, so doc.install "docs/agent-runbook.md" is preserved
• require HOMEBREW_TAP_TOKEN and update the tap before creating the GitHub Release, avoiding a successful-looking release where Homebrew still lags

Validation

• swift test --parallel
• swift build -c release
• ./.build/release/vch version
• ./.build/release/vch runbook --json
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/release.yml")'
• local simulation of the tap formula rewrite confirmed doc.install "docs/agent-runbook.md" and runbook formula tests are preserved

Release step after merge

Push tag v0.8.1 from the merged master commit to trigger the release workflow.

[Maples7]

Closes #108